Ultrasonic welding apparatus

ABSTRACT

An ultrasonic welding apparatus and method appropriate for welding sheets of plastic including oriented polyesters such as polyethylene terephthalate film, including means for chilling the workpieces while they are being compressed by the ultrasonic tool.

United States Patent 1 1 1 1 3,717,539 Roberts 1451 Feb, 20, 1973 54] ULTRASONIC WELDING APPARATUS 3,438,428 4/1969 51111111111111 et al. ..l56/498 2,452,211 10/1948 11051111111111 .228/1 [75] Inventor: Ham'd Rberts1 Da'lasjex- 3,405,024 10/1968 Attwood etal ..l56/73 [73] Assignee: E-System,lnc., Dallas Tex.

Filed: Aug. 28, 1969 App1.No.: 871,211

Related U.S. Application Data Division of Ser. No. 732,216, May 27, 1968, Pat. No. 3,488,240.

U.S. Cl. ..l56/498, 156/73, 156/580 Int. Cl. ..B32b 31/20 Field of Search ..l56/73, 498, 580; 228/1 References Cited UNITED STATES PATENTS 4/1970 Lawrence ..l56/73 Primary ExaminerBenjamin A. Borchelt Assistant Examinerl-l. J. Tudor AttorneyJames D. Willbom [57] ABSTRACT An ultrasonic welding apparatus and method appropriate for welding sheets of plastic including oriented polyesters such as polyethylene terephthalate film, including means for chilling the workpieces while they are being compressed by the ultrasonic tool.

2 Claims, 9 Drawing Figures ULTRASONIC SIGNAL GENERATOR PATENTEDFEBZOIQB I 53717. 539

SHEET 10F 2 l3 l3 r ULTRASONIC uLTRAsONIc SIGNAL T|MER SIGNAL GENERATOR l2\ y GENERATOR I6 22 3 3 '9 l7 EA? 2| 2| HIGHEST POINT OF TOOL TRAvEL REsT POSITION LOwEsT POINT .Ol08- .003- OF TOOL TRAVEL 7 g I SHIM l7 SHIM "L. MYLAR I ----wELO zONE-- L MYLAR 10039 L ,OIOG- .OOG2

BEFORE WELDING AFTER WELDING FIG 3 FIG 4 HAROLD D. ROBERTs, FIG 8 INvENTOR PATENTEB FEB 2 01973 FIG 6 0:.OIOO Lu COMPRESSION RELAXATION AND HEATING TEMPERATURE .ooso- SHEET 2 BF 2 BEGINING END OF OF WELDING WELDING LL! 0: D RT RT LLI L w U TlME B N SOAKING BEGINING END OF OF WELDING WELDING PRIOR ART J TIME TOP OF SHIM AND TOOL TRAVEL\ BOTTOM OF SHIM t L J r COMPRESSION RELAXATION AND HEATING AND COOLING LAST CYCLE AND COOLING HAROLD D. ROBERTS,

INVENTOR ATTORNEY ULTRASONIC WELDING APPARATUS This is a division of application Ser. No. 732,216, filed May 27, 1968, now U.S. Pat. No. 3,488,240.

The high tensile strengths of oriented polyester sheets, for example, polyethylene terephthalate, has made such materials popular for a variety of applications, perhaps the most common of which is as a base for photographic film. Of the sheets that are commercially marketed, one of the most widely known is that marketed by E. l. duPont de Nemours & Co. under the trade name Mylar; and, for convenience herein, the name Mylar is frequently used as a shorthand way of referring to all such oriented materials. The high tensile strength of Mylar is achieved by stretching sheets of the material approximately percent in its width direction and approximately 30 percent in its length direction while the material is held at somewhat above 300F. The mechanical stretching aligns the molecules and produces excellent properties in the length and width directions, although rather unimpressive properties in the thickness direction. The material is cooled to room temperature while it is being held in tension so that substantially all of its strength is retained. If the unrestrained material is ever heated back to a temperature in excess of 300F, the heat relaxes the mechanically induced orientation and causes the material to shrink; and, as might be expected, the material thereupon loses most of its strength.

Since oriented polyester film is sensitive to heat, it naturally would be hoped that adhesive joints would be a solution to the joining problem. Unfortunately, polyester fih'ns are not readily bonded by adhesive techniques to most materials, including themselves. It is suspected that this difficulty is the result of the material having what is sometimes referred to as a platelet-like molecular structure. One class of materials, of which polyurethane is an example, can, however, be bonded to Mylar. Hence, it has been common to employ as a substratum in photographic film, for example, a very thin layer of polyurethane (or a material which is chemically related to polyurethane), the substratum naturally being compatible with both the polyester base and the emulsion which carries the light-sensitive materials. Thus, prior art joining techniques have had to content with not only the base material but also the particular emulsion and substratum which is employed. The emulsion and substratum can, of course, be scraped off of the base to present a clean and bare surface on the ends of both pieces to be joined; but an adhesive joint of pure Mylar to pure Mylar if obtainable at all has not heretofore had the strength which was desirable. Therefore, such cleaned joints have usually been avoided heretofore in favor of making emulsion-to-emulsion or emulsion-to-base joints.

Because of the difficulty of achieving adhesive bonds, ultrasonic welding has been attempted with varying amounts of success. The basic technique of ultrasonic welding has been described by F. P. Alles in an article entitled Ultrasonic Splicing of Polyethylene Terephthalate Films published in December, 1961, in Volume 70 of the Journal of the Society of Motion Picture and Television Engineers. It is suspected that whatever success has been achieved with ultrasonic welding has been due, at least in part, to confining the heat-affected zone to a relatively small area, such that the thennal rise in the material as a whole is kept to a minimum. While the heat affected (and therefore weakened) material has perhaps been minimized by the use of prior-art ultrasonic techniques, it is believed that such previously employed techniques can never completely eliminate the effects of re-heating some of the material back to the temperature at which it was manufactured.

The joints or splices which have been achieved in photographic film in the past, wherein some emulsion is deliberately left between two overlapping pieces of base material, have been reported to be from 65 to percent of the strength of the original material. Attempts to experimentally reproduce such reported results, however, have resulted in far more joints near the 65 percent end of the range than near the 100 percent end. Too, for some unknown reason, the results have sometimes been erratic, such that identical machine settings, etc., produced a weld in some cases and produced no weld in others. The inability to consistently reproduce high-strength joints with at least some emulsion trapped between two pieces of base material raises the inference that the emulsion, has a deleterious effect on the finished joint. Hence the ironical situation apparently existed in the past that at least same emulsion was necessary to produce any joint at all, but the presence of the emulsion in the finished joint eventually led to fatigue failure at the joint, if a tensile failure did not occur first.

It is believed that very few persons, if any, recognized the fact that it is apparently the emulsion on photographic film that contributes to the inability to repeatedly make good welds or splices. Thus, the few publications that have described ultrasonic splicing techniques have either been silent on the presence of the emulsion or merely stated that a cleaning step to remove the emulsion is unnecessary. As will be explained hereinafter, however, it has been found that better welds can be achieved if the emulsion and substratum are first removed from photographic film than can be achieved when these materials are left in place.

Accordingly, it is a major object of this invention to provide a novel method of welding plastic films with ultrasonic energy.

An additional object is to provide a method of joining an oriented polyester film to itself without the use of an intermediate material.

Another object is to provide a novel ultrasonic welding apparatus.

A further object is to provide a method of welding Mylar to itself and to other oriented polyesters.

Yet another object is to provide a method of splicing photographic film wherein the splice 'is stronger than the base material.

Other objects and advantages will be apparent from the specification and claims and from the accompanying drawing illustrative of the invention.

In the drawing,

FIG. 1 is a diagrammatic representation of an ultrasonic welding apparatus which is typical of the prior art;

FIG. 2 is a diagrammatic representation of an ultrasonic welding apparatus arranged in accordance with a preferred embodiment of the invention;

FIG. 3 is an enlarged elevation view of small segments of the materials being welded, with the various tool positions being shown for reference purposes;

FIG. 4 is a cross-sectional view of a joint representative of those achieved with the invention;

FIG. 5 is a time-temperature plot of a segment of material being welded in accordance with the invention;

FIG. 6 is a time-temperature plot of a segment of material being welded in accordance with prior art welding methods;

FIG. 7 is a graphical representation of the sequence through which a given small section of the materials pass as they are welded;

FIG. 8 is an isometric view of a small segment of photographic film before the gelatin is stripped off in preparation for welding; and

FIG. 9 is an isometric view of the film segment shown in FIG. 8 after the segment has had its gelatin top coat removed by sandblasting.

With initial reference to FIG. 1, an ultrasonic welding apparatus 10 arranged in accordance with prior practice is shown diagramatically. The apparatus includes a welding tool 11 which is caused to oscillate along its longitudinal axis by the transducer 12 affixed to the top of the tool. The transducer 12 received energy from a source such as the ultrasonic signal generator 13 and converts it into linear movements which are transferred to the tool 1 1 in a well-known manner. During downward excursions of the tool 1 1, the overlapped edges of the workpieces 14 are repeatedly compressed against the rigidly supported anvil 15, such that sufficient heat is imparted to the workpieces to fuse the pieces together. The workpieces 14 are typically moved with respect to the tool 1 1 with the aid of a table or the like (not shown) which travels beneath the tool at a known rate.

With reference to FIG. 2, a welding apparatus 16 constructed in accordance with the teachings herein includes substantially the same elements as those shown in FIG. 1, but with added elements for chilling the workpieces at the conclusion of an excursion of the tool tip against the workpieces while said workpieces are still being compressed by the tool, tip. These added elements may, of course, take most any reasonable form, but it has been found most convenient to employ a heat-conductive element 17 (such as copper shim) which is placed in contact with the lapped workpieces 14, together with a means for chilling the exposed surface of the shim. A convenient chilling means has been found to be a conventional bottle of pressurized carbon dioxide 18, with a suitable valve 19 which controls the passage of CO through suitable tubes 20 and nozzles 21 that direct the gas directly onto the shim 17.

The nozzles 21 have orifices that are suitably sized so that cooling (which is caused by expansion of the pressurized gas) occurs at the orifices and not at the valve. A timer 22 and a suitable connection represented by the broken line 23 are employed, if desired, to achieve what may be described as a substantially automatic welding process, wherein the welding energy is imparted to the workpieces in timed relation to the discharge of gas onto the work area.

In operation of the apparatus 16, the height of the tool tip 24 is preferably adjusted so that its neutral or at rest" height is slightly less than the stacked height of the pieces of material to be welded 14, plus the heatconductive element 17. An example of a suitable adjustment is graphically protrayed in FIG. 3, wherein two pieces l4, 14 of Mylar are shown with a copper shim 17 measuring 0.003 inch placed in contact with the top surface of the stacked Mylar. The thickness of the shim 17 is not critical, but it must be measured in order to initially adjust the tool to a suitable height. The left portion of the figure represents the stacked height of small segments of the elements 14, 14 and 17 before welding is commenced, and the right portion represents the same segments after welding. Examination of the welded pieces 14, 14 after welding would likely reveal a cross-section like that shown in FIG. 4.

The period of actual welding is typically established by keeping the tool 11 fixed and moving the film pieces l4, l4 beneath it. Thus, the film pieces 14, 14 are usually placed in a jig so that their ends overlap an appropriate distance, e.g., 0.030 inch, and then the jig is attached to a movable table or anvil which is programmed to move underneath the tool 1 1 at the proper time. The rate of travel of the table or anvil 15 beneath the tool 11 of course determines about how many cycles of compression and relaxation that given segments of the workpieces 14, 14 will experience. Table travel is typically set at about one-half inch per second, and the signal generator 13 usually produces a signal having a frequency of about 20,000 cycles per second; hence it is believed that each small segment of the pieces l4, l4 experiences from several hundred to a few thousand compression cycles, depending on the size of the tool tip 24.

It is, of course, the repeated compression and relaxation of the workpieces 14, 14 underneath the oscillating tool tip 24 that introduces heat into the pieces, such that eventually they become fused together. Such a heating step is obviously old; but what has heretofore not been taught is the step of chilling or quenching the fused portions of the workpieces while said portions are still under compression. This is accomplished by continuously removing heat from the workpieces 14,14, including that time during which compression is adding heat to the workpieces, so that the fused portions are immediately chilled at the end of a compression step.

It has been found convenient to effect the required cooling by directing a jet of cooling gas onto the weld zone, although other cooling means could probably be employed with equal success. For example, an anvil would be provided with a chamber through which a cooling liquid is circulated, much like the water jacket through which water is circulated in a familiar internal combustion engine. If a cooling chamber could be included in the tool tip 24 without altering the resonance characteristics of the tool 11, filling such a chamber with a liquid at cryogenic temperatures would also be a satisfactory manner of chilling the workpieces at the conclusion of a compression cycle. However, an apparatus 16 which includes the simple shim l7 and a gas bottle 18 at 600 psi or so, has been found to be convenient to use, and is much more economical to assemble than would be a special jacketed anvil, etc.

To insure that the chilling will begin immediately after the first heating cycle is complete, it is advisable to direct sufficient cooling gas onto the shim 17 to chill the shim and the workpieces before any actual welding is begun. If CO is employed for cooling a flow rate of about 2 liters per second (at atmospheric pressure) against the shim 17 which is sufficient to cause frost to immediately appear on the shim and which is maintained for about three seconds) has been found to be quite satisfactory. It should be apparent, however, that if a material other than copper is employed as the heatconducting element 17, an alteration in initially chilling or soaking time might be required, the change in time being proportional to the difference in the thermal conductivity of copper and the substituted material.

The initial cooling or soaking of the workpieces is represented in the left-hand portion of FIG. 5. When the tool tip 24 compresses the overlapped workpieces 14, 14 the first time, a certain amount of heat will be generated within the workpieces, and the time-temperature curve will have an upward slope. The exact amount of heat imparted to the workpieces during a single compression cycle is not readily susceptible of measurement, so the curve in FIG. 5 has been left dimensionless in order to avoid giving the impression that precise information is available on what happens during a given one of the thousands of cycles. Based on the heating and cooling means provided, however, it can be concluded that the time-temperature curve of the workpieces would have the peaks and valleys shown in FIG. 5 during the welding operations. The appreciable difference between the welding method taught herein and prior art methods can be recognized when FIG. 5 is compared with prior welding techniques depicted in FIG. 6.

Like the exact amount of heat added during a single compression cycle, the exact maximum temperature that the workpieces reach during heating is not known with certainty. It is reported, however, that Mylar melts (using the term very broadly) somewhere above 450F at atmospheric pressure. Hence a joint between two pieces of Mylar that has been achieved with ultrasonic welding has almost certainly been at or near a temperature of about 450F. Thus, it has been exposed to temperatures appreciably in excess of that temperature which is slightly above 300F, and at which breakdown of the molecular orientation begins to occur. The fact that the maximum temperature is not known with certainty is of no consequence, however, since it is known that welds are achievable with welding devices that are commercially available when such devices are combined with a chilling means in accordance with the teachings herein. Those skilled in the art will find no difficulty, therefore, in applying the teachings herein.

While the peaks and valleys of the time-temperature curve of FIG. 5 cannot be assigned precise values with any assurance, the basis on which the curve is drawn is supported by examination of what happens to a small segment of the workpieces 14, 14, as they pass through the weld zone, shown in FlG. 7. The first contact between the tool tip 24 and the stacked elements 14, l4, 17 is made when the stacked elements are, in this example, about 0.0108 inch high. As the tool tip 24 reaches the lowest point of its oscillations, the Mylar pieces have been compressed to a combined height of about 0.0032 inch. The copper shim 17 is relatively rigid compared to the Mylar, and compression in it can be disregarded as negligible. At the end of the first compression stroke, it is believed that the resiliency in the Mylar pieces 14, 14 will cause them to spring back to very near their original height as the tool tip 24 is withdrawn. That is, it is practically certain that first compression cycle does not impart enough heat to bring the interface to a fusion temperature; and there is little reason to suspect that much if any permanent set is imparted to the materials in the first cycle. When the pieces 14, 14 begin to rebound at the end of each compression stroke (as the tool starts its upward movement), the chilling effect of the continuously flowing and cold CO gas is realized, and, it is believed, the respective molecular structure of each of the pieces is restrained from taking on a random orientation, i.e., it is fixed, while the pieces are still under compression. The tool 11 has been observed to follow what appears to be a nearly perfect sinusoidal path in moving between its highest and lowest points of travel, so that the first compression cycle is repeated in the plurality of cycles which follow.

The last compression cycle to which segments of the Mylar pieces 14, 14 are subjected, compresses them and the shim 17 to the same depth as the first cycle, but the thousands of repetetive compression steps (with the attendant heat) eventually introduce a permanent set in the pieces, and the last upward movement of the tool 11 leaves the final height of the welded Mylar pieces at about 0.0062 inch. Such a thickness in the welded joint is not so much greater than the original thickness as to pose an obstacle in many applications; but it should be apparent that the final thickness could be reduced by such steps as, for example, lengthening the weld time so as to increase the number of compression cycles to which each segment is subjected, etc.

The strength of joints achieved when the chilling step described herein is employed has been found to exceed the original tensile strength of the Mylar sheet. Testing of the joints has always produced failures well outside of the weld zone, so that it may be concluded that not only is a sound joint consistently produced but also weakening of material adjacent the weld zone (a problem which is very pronounced in prior art welds) is reduced if not eliminated. This benefit can be more fully realized by making the shim larger in area than that portion of the tool tip which bears against the shim, so that the shim extends beyond the tool and cools an area appreciably larger than the weld zone. Furthermore, cooling the material adjacent the weld zone so as to keep the heat-affected region small, makes possible relatively large power imputs when desired, e.g., on the order of 300 watts, as compared with 50-100 watts input in most reported devices.

The welding process described thus far has considered only bare Mylar, since this is apparently the most difficult material in which to achieve a sound joint. Bare Mylar, howeyer, does not have nearly as many commercial uses as Mylar which is coated with an emulsion or the like. Thus, photographic film usually has, in ascending order, a base of Mylar (which comprises about three-fourths of the film thickness), a substratum such as a polyurethane resin, an emulsion carrying silver halide particles, and a final gelatin overcoat; beneath the base is frequently an antihalation coating to prevent any light which passes through the transparent base from the reflecting back into the lightsensitive emulsion. The most widely known and commercially available film is perhaps duPonts Cronar film. This film is said by one source to employ an oriented polyester base other than Mylar and a substratum other than polyurethane; but if such other materials are indeed employed, they are so closely related, respectively, to Mylar and polyurethane that relatively simple laboratory tests will not distinguish between the materials. Perhaps more significant with respect to the instant disclosure, the Cronar base behaves like Mylar when it is being welded. Hence, for the purposes of teaching a method of joining film strips, any difference that in fact exists between Mylar and the Cronar base can be disregarded, and a technique that is applicable for pure Mylar will be understood to be applicable for Cronar also.

As suggested earlier, it is believed that much stronger bonds are achieved in photographic film when all of the emulsion is first removed so that "a bond is achieved between the base material of the respective pieces, with no impurities to contribute to immediate or subsequent failure In the past, there have been two known ways of removing emulsion, i.e., to chemically remove it and to scrape it off with a knife or the like. The chemical technique utilizes any known solvent for the emulsion, including, for example, a commonly known household bleach such as Clorox, to dissolve the emulsion and wash it away from the base. Wicking is a problem with this technique, however, and a sharp boundary in the remaining emulsion is difficult to obtain. Scrapping the emulsion off with a knife can produce a sharp boundary, but it almost inevitably results in scratches that are believed to lead to premature failure. That is, Mylar joints made with materials having scrapped surfaces have been experimentally found to exhibit poor fatigue properties, and it is believed that the scratches are a major contributor to observed failures.

It has been found, however, that the emulsion can be removed with no deleterious effects by sandblasting it off of the base. A convenient apparatus for accomplishing this is an abrasive unit such as the Model Csold by S. S. White Dental Manufacturing Co., New York, N. Y., which is traditionally sold to dentists for laboratory work, preparing cavities for subsequent filling, etc. Using abrasive powder such as aluminum oxide having an average particle size of about 50 micron, and a propelling gas at an operating pressure of about 75 psi, the emulsion is readily eaten away when a nozzle directing the blast is moved over the material about as fast as a person moves a pencil in writing his signature on paper. The duration of the blast is not what might be described as critical, however, since the Mylar is resilient enough to yield to some extent under a continuous blast, and it is not readily removed. The relatively brittle emulsion, on the other hand, is easily removed by the sand blast, as is the substratum. Adequate removal of unwanted materials can be determined by inspection of the Mylar base which takes on a frosty appearance much like ground glass when the sand blast has removed all of the unwanted materials and then has impinged directly on the bare Mylar for a brief period.

FIG. 8 represents a segment of photographic film in which the three principal components are shown, viz., the base 25, the substratum 26, and the light sensitive emulsion 27. For convenience, the film can be advantageously clamped in a jig (not shown) in which only that portion to be stripped is exposed, the remainder of the film being protectively covered. A

nozzle which directs the abrasive-laden stream of gas is then passed over the exposed film to remove the substratum 26 and emulsion 27, which leaves the film with a configuration like that shown in FIG. 9. When photographic film having the aforementioned antihalation backing is to be spliced, the mating surfaces of the respective ends should be stripped in order to achieve a joint whose strength is as high as possible. That is, the gelatin emulsion on the top of one piece of film and the gelatin emulsion on the bottom of the other piece of film should be stripped, and then the cleaned surfaces should be placed in contact beneath the ultrasonic tool tip.

While the welding method described herein has been particularly directed to the problems of welding oriented polyesters, it should be obvious that the technique is suitable for other plastics whose respective properties are largely' attributable to an oriented molecular structure. Similarly, only one embodiment of the apparatus has been described in detail herein and shown in the accompanying drawing; but it will be evident that various modifications are possible in the arrangement and construction of its components without departing from the scope of the invention.

What is claimed is:

1. In an ultrasonic welding apparatus for welding together two or more sheets of molecularly oriented plastic material having a signal generator whose output is converted into periodic and substantially linear excursions of a tool tip, the excursions of the tool tip being utilized to compress the overlapping workpieces such that the heat is added thereto, the improvement comprising:

a heat conductive element in contact with the overlapping workpieces positioned between the tool tip and the overlapping workpieces for heat removal therefrom, 1

cooling means for cooling the heat conductive element at the conclusion of an excursion of the tool tip against the workpiece while the workpieces are under compression by the tool tip, and

timing means for controlling said cooling means in a timed relationship with the application of welding energy to the tool tip by said signal generator to cool the weld after removal of the welding energy.

2. In an ultrasonic welding apparatus for welding together two or more sheets of molecularly oriented plastic material having a signal generator whose output is converted into periodic and substantially linear excursions of the tool tip, the excursions of the tool tip being utilized to compress overlapping workpieces such that heat is added thereto, the improvement comprising:

a heat conductive element in contact with the overlapping workpieces positioned between the tool tip and the overlapping workpieces for heat removal therefrom,

a source of cooling gas,

a nozzle positioned for directing a fluid stream onto the heat conductive element,

means for connecting the source of cooling gas to said nozzle, and

means for timing the relationship of gas discharge through said nozzle onto said heat conductive element with the application of welding energy to the tool tip from said signal generator to cool the weld after removal of the welding energy. 

1. In an ultrasonic welding apparatus for welding together two or more sheets of molecularly oriented plastic material having a signal generator whose output is converted into periodic and substantially linear excursions of a tool tip, the excursions of the tool tip being utilized to compress the overlapping workpieces such that the heat is added thereto, the improvement comprising: a heat conductive element in contact with the overlapping workpieces positioned between the tool tip and the overlapping workpieces for heat removal therefrom, cooling means for cooling the heat conductive element at the conclusion of an excursion of the tool tip against the workpiece while the workpieces are under compression by the tool tip, and timing means for controlling said cooling means in a timed relationship with the application of welding energy to the tool tip by said signal generator to cool the weld after removal of the welding energy. 